In an ideal case, where a particular image is to be viewed by an audience, each area of a screen (or other projection-receiving surface) should reflect to the audience the desired color and intensity of light at the desired time. However, there are physical constraints that cause light pollution, where light pollution includes the reflection or re-reflection of light from one or more of those screen areas that includes light that was not intended to be reflected from that point. Typically, this “light pollution”, an unintended leakage of light due to some physical property of the display system, might be due to curvature of the screen or ghosting in stereoscopic 3D display systems, wherein light intended for one eye is visible to the other eye and combines with the light that actually was intended for the other eye.
Light pollution in 3D display systems results in “ghosting”—an object that is slightly displaced between the two stereoscopic images in order to convey to the eyes its depth appears as a “ghosting image” when portions of on eye's image leak over to the other eye's image. In some cases, the light pollution is easily modeled. For example, it might be determined by empirical or other methods that a particular 3D display system leaks 10% of the right eye's image onto the left eye's image and 10% of the left eye's image onto the right eye's image due to imperfect polarization of projected or reflected light.
In a conventional approach to deghosting, the right eye's image is composited with a negative 10% of the left eye's image and the resulting composite is displayed. However, there is a problem for pixels in the right eye's image that are supposed to be darker than the light pollution from the left eye (or vice versa). This corresponds to attempting to subtract more light than is available, leaving the resulting composite image with negative light values. Of course, since negative light is impossible to achieve in any type of physical display device, the composite image retains some of the light pollution.
To deal with the remaining light pollution, some producers will modify the “black level” so that the darkest points of an image are not totally black, thus allowing for more subtraction. However, this reduces the dynamic range of the displayed images.
Another approach is to carefully direct and edit an image sequence that is being presented so that bright objects in the foreground of a scene do not appear in front of dark objects, thus lessening the instances of negative light values in the composite image. Unfortunately, this may unduly constrain directors, cinematographers, artists, etc. when they desire to use a large dynamic range of light in close proximity.
Yet another approach is to ignore the remaining light pollution and have the viewers mentally deal with the remaining ghosting. However, this can lead to optical fatigue, especially with feature length films.
The goal of most, if not all, display systems is to provide viewers with the highest possible quality viewing experience. Many factors determine the overall viewing quality, but an important one is quality reduction due to unintended light pollution onto (portions of) the intended image.
It is important to reduce the cognitive load on the viewers of the ghosting that cannot be eliminated by simple subtraction.